1. Field of the Invention
The invention relates to communications systems in general and more particularly to devices and/or techniques which align a receiver with a transmitted serial data stream.
2. Prior Art
The use of serially communicating devices is well established in the communications industry. Most of these devices include standard or off-the-shelf modules which, in the case of a transmitter, receive data from a sub-assembly of the device and process the data that is transmitted onto the communications medium. In the case of a receiver, the modules receive the serial data stream, process it and deliver the data to the sub-assembly in the device. These modules are generically known as Serial Communications Controller or Universal Synchronous/Asynchronous Receiver/Transmitter (USART). Z8030/Z8530 (marketed by Zilog, Inc.), Intel 82530 (marketed by Intel Corporation) and SCN26562 (marketed by Signetics Corporation) are examples of serial communications controllers.
One of the functions which a serial communication controller (SCC) must provide is to synchronize the receiving circuitry with the bit boundary of the transmitted data stream. The prior art SCC requires that the first transmission of received data be on a defined bit/baud cell boundary. More particularly, some of the prior art SCCs require that the first edge seen by its receiver for FM encoding be on a bit cell boundary. If this is not so, these devices set interrupt bits and re-initiate the bit/baud synchronization process again. This requires microcode intervention each time a serial data exchange is initiated, or any time data is corrupted due to poor reception. The reinitiated process requires that the receiving station request the transmitting station to start at an idle state with no transitions and then start transmitting data with the defined bit/baud restriction. The need for microcode intervention and the retry procedure unnecessarily increase product cost and reduce data throughput.
Another prior art synchronization technique requires that sync bits be inserted in the transmitted data stream. The receiver uses these bits to synchronize the receiver with the data. U.S. Pat. No. 4,613,979 describes a synchronizer that uses special bits in the data stream for synchronization.
There are conditions under which the prior art synchronizing apparatus does not work. One example of such conditions occur if the digital phase-lock loop (a necessary sub-assembly of the USART) drifts off its intended point due to data corruption. This would require resynchronization and the prior art USART may resynchronize on the wrong baud cell boundary and not see its intended closing sequence.